1. Field of the Invention
The present invention relates to a light source unit emitting a light beam, an optical scan apparatus scanning a scan surface with a plurality of light beams and an image formation apparatus incorporating such an optical scan apparatus.
2. Description of Related Art
In the prior art, an image formation apparatus forming images by Carlson process is known. For example, Japanese Laid-open Patent Application Publication No. 2002-341273 discloses such an image formation apparatus which scans the surface of a rotating photoconductive drum with a light beam to form an electrostatic latent image, visualizes the electrostatic latent image as a toner image, and fuses the toner image on a sheet of paper as a recording medium to form an image. This type of the image formation apparatus has been popularly used in simple printmaking for an on-demand print system. There has been increasing demand for high-density images and high-speed image outputs.
Japanese Laid-open Patent Application Publication No. 2003-211728, for example, discloses an image formation apparatus which has a light source such as a vertical cavity surface emitting laser (VCSEL) array with plural light emission portions two-dimensionally, monolithically arranged and is capable of concurrently scanning a scan surface with plural light beams.
Such a vertical cavity surface emitting laser array used in the image formation apparatus is composed of the light emission portions on a chip contained in a package, which is made of a ceramic material or the like. For mounting the light emission portions on a circuit board by soldering, the mounted light emission portions are not uniform in height. With such non-uniformity in height, the surface of the package is inclined relative to the surface of the circuit board. As a result, positional relations between the individual light emission portions and the circuit board surface may not be the same.
In such a light source unit the positions of optical elements such as a coupling element (lens) are often determined on the basis of the circuit board surface. In this case, there will be a problem that the positional difference between each light emission portion and the circuit board surface leads to a positional difference between the optical elements and each light emission portion.
In order to form images in high quality by an image formation apparatus, the light source unit need be disposed at a precise position relative to the optical system. For example, the light emission portions have to be positioned precisely relative to the coupling element at unit of several μm. With a low precision, beam spot sizes or pitches of light illuminating photoconductive drums are adversely affected, resulting in significantly deteriorating image quality.
For prevention of such a problem, Japanese Laid-open Patent Application Publication No. 2004-6592 discloses a method for positioning the package surface relative to the optical elements in a semiconductor laser unit by elastically bending the circuit board to bring the package into contact with the surface of a support member by pressure.
However, such a method still has a problem that enforcedly bending the circuit board causes solder of electric components mounted thereon to be peeled off, which may adversely accelerate deterioration of the laser unit over time, for example.
Furthermore, there is another problem in fixation of the light emission portions that since they are fixed to the package by an epoxy adhesive or the like, it is hard to precisely position them on the package even by use of a jig or the like. It is not possible to accurately set the position of the light source relative to the optical system.